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Received Feb 8, 2018; Revised Mar 16, 2018; Accepted Apr 3, 2018
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1. Introduction
Chronotype refers to preferences for timing of sleep and wakefulness [1, 2]. Chronotype reflects individual differences in circadian rhythms [3]. Determining chronotype is important as it affects many aspects of behavior and health, including sleep duration [4], cognitive performance [5], and psychopathology [6]. Determining chronotype is also helpful for the diagnosis and treatment of circadian rhythm sleep-wake disorders [7]. Objective measures can be used to estimate the relationship between the time point of internal biomarkers, such as dim light melatonin onset (DLMO) and core body temperature, and a zeitgeber (environmental cue, such as daylight) [8]. This relationship between the time points of the internal marker and external cue is called phase angle or phase of entrainment [9]. These biological measures exhibit high degrees of validity and reliability in assessing the circadian phase angle, but they are expensive and labor intensive and require high degree of subject participation [10]. Actigraphy may be used as an additional tool to estimate circadian phase angle [11]. Actigraphy is a noninvasive, objective measure of sleep that is reliable and valid compared to polysomnography in the measurement of sleep schedule and duration. However, it must be worn continuously at nights and requires participants’ compliance.
Subjective measures of chronotype include questionnaires that ask about an individual’s expressed preference for morning or evening activities. These questionnaires, such as the Morningness-Eveningness Questionnaire (MEQ) and Composite Scale of Morningness (CSM), can either classify individuals into categories (morning type, evening type, or neither type) based on such preferences or report chronotype on a continuum as a dimensional construct where an individual can have greater “morningness” or “eveningness” depending on where they fall on the spectrum. One such dimensional measure is the Munich Chronotype Questionnaire (MCTQ), a self-report questionnaire that collects information on an individual’s habitual sleep schedule during work days and free days [12]. It estimates phase of entrainment by setting a reference point based on the reported sleep-wake cycle rhythm [8]. The reference point is the midpoint of sleep on free...